1. Field of the Invention
The present invention relates generally to a discharge-lamp lighting apparatus for lighting discharge lamps such as cold cathode fluorescent lamps (CCFLs), and particularly, to a technique of simultaneously driving a plurality of CCFLs.
2. Description of the Related Art
FIG. 1 is a view showing a configuration of a discharge-lamp lighting apparatus according to a related art. This apparatus includes a first DC power source E1 connected in series with a first switching element Q1 and a second switching element Q2. The first and second switching elements Q1 and Q2 are turned on and off in response to control signals from a first control circuit 10a. The first control circuit 10a conducts PWM control, phase control, frequency control, or the like to control the on/off operation of the first and second switching elements Q1 and Q2.
The second switching element Q2 is connected in parallel to a quasi-voltage-resonance capacitor C6. The element Q2 is also connected in parallel to a series circuit that includes a primary winding P1 of a first transformer T1 and a current resonance capacitor C4. The first transformer T1 has a leakage inductance Lr1 for managing resonance operation.
A secondary winding S1 of the first transformer T1 is connected, through a resistor RS1, in parallel to a series circuit that includes a cold cathode fluorescent lamp (hereinafter referred to as CCFL) 11a and a ballast capacitor C11 and a series circuit that includes a CCFL 11b and a ballast capacitor C12. The resistor RS1 is a current detection resistor to detect a current. A current detected by the resistor RS1 is fed back to the first control circuit 10a on the primary side. According to the signal fed back from the resistor RS1, the first control circuit 10a controls on/off periods of the first and second switching elements Q1 and Q2, thereby controlling an AC voltage applied to the primary winding P1 of the first transformer T1.
FIG. 2 shows a voltage-current characteristic of a typical CCFL. The CCFL has a negative resistance characteristic that a current (lamp current) increases as an applied voltage (lamp voltage) decreases. To relieve the negative resistance characteristic, the discharge-lamp lighting apparatus inserts an impedance element in series with a CCFL. The impedance element must have a sufficient value to absorb the negative resistance characteristic of the CCFL. When driving a single CCFL, the discharge-lamp lighting apparatus uses the leakage inductance Lr1 of the first transformer T1 as the impedance element.
When simultaneously driving a plurality of CCFLs, simply connecting the CCFLs in parallel with one another causes a problem that a CCFL that is first turned on triggers a voltage drop due to impedance to prevent the other CCFLs from being turned on. To avoid this problem, an impedance element is inserted in series with each CCFL. In the example shown in FIG. 1, the impedance elements are the ballast capacitors C11 and C12. With the ballast capacitors C11 and C12, a voltage applied to the secondary winding S1 of the first transformer T1 becomes free from the ON or OFF state of any CCFL, and therefore, all CCFLs are surely turned on.
Another example of the discharge-lamp lighting apparatus is a multi-lamp drive system disclosed in Japanese Unexamined Patent Application Publication No. 2003-31383. This multi-lamp drive system drives a lamp set consisting of first and second lamps. The system includes a drive circuit for converting a DC signal into an AC signal, a transformer whose primary side is electrically connected to the driver circuit and whose secondary side provides an AC power source, and a current balance circuit electrically connected to the lamp set to balance currents passing through the first and second lamps. The current balance circuit has a core, a first winding electrically connected to the first lamp, and a second winding electrically connected to the second lamp. The first and second windings are wound around the same core and have the same number of turns.